Genetically obese ob/ob mice with B16 melanoma tumors survive longer than do lean littermates. Despite their increased survival, ob/ob mice become anorexic and lose weight during tumor growth, whereas lean littermates become hyperphagic and gain weight. Healthy ob/ob mice possess higher endorphin levels than do lean mice, and opiates have been reported to depress cell growth, increase food intake, and facilitate development of learned food aversions. Three experiments are proposed in the present request. Experiment 1 will examine whether the anorexic response of ob/ob mice during tumor growth reflects an enhanced propensity to develop aversions for diets associated with illness. Healthy ob/ob and lean mice will be injected with one of two doses of lithium chloride (or saline) following consumption of a palatable saccharin solution, and will be tested for differences in acquiring and extinguishing a conditioned taste aversion. Experiment 2 will determine how the tumor survival of lean mice would be altered if food intake were restricted to a degree approximating that occuring in ob/ob mice. Lean mice will (a) be prevented from increasing their food intake or (b) will have their intake reduced by an amount equivalent to that occurring in obese mice, and effects upon tumor growth, metastasis, and survival will be examined. Experiment 3 will investigate whether the differences in tumor survival and food intake between ob/ob and lean mice persist when endogenous opiates are blocked. Ob/ob and lean mice will be injected daily with one of two doses of the opiate antagonist naltrexone (or saline), and effects upon tumor growth, metastasis, survival, and food intake will be evaluated. Obese humans appear highly resistant to some cancers, and the ob/ob mouse may provide a model for investigating this phenomenon. The proposed 3-point plan will explore some physiopsychological aspects of cancer anorexia in ob/ob and lean mice.